This invention generally pertains to the field of digital data recording. In particular, the invention focuses on a tracking method and track format for optical data recording.
An optical record carrier, also referred to as an optical disc, is typically preformatted with a spiral track, usually spiraling outward with a fixed track-pitch. One entire revolution of the spiral is typically referred to as the track. Tracks are typically divided into sectors. Sectors are where digital information is stored in the form of marks and pits, and from which information will be read or retrieved. A track of a spinning disc is read via the light reflected from the marks and pits, which is processed into an electrical signal.
(While pits and marks can refer to, respectively, the pre-formatted data and the information that is stored by the user on the disc, such distinctions in nomenclature is not always made. Thus, this Application will generally refer to "marks" as any information (pre-formatted information, information written by the user, etc.) on the disc.)
For a disc having multiple concentric tracks, the direction at a point along a track is referred to as the tangential direction, while the direction normally across the tracks (i.e., from the center of the disc outward) is referred to as the radial direction.
The pre-formatted data on the tracks of the disc include a servo field or region. Marks in the servo field are used to maintain the laser's radial alignment with respect to the particular track that is being read. The reading and processing of marks in the servo field provide an indicia of the laser's radial position with respect to a track and allow for it's correction. Thus, proper reading and processing of the other marks (such as the information stored by the user) on the track is better assured.
An example of a prior art track format is given in FIG. 1. The format is representative of the one used in the LM4000 media of Philips Laser and Magnetic Storage. FIG. 1 gives a simplified schematic of the format of certain marks in the servo field. It is noted that although the four tracks shown in FIG. 1 (labeled Tracks N to N+3) are concentric with the center of the optical disc they reside on, the part of the servo field shown in FIG. 1 is a small portion of the overall track and thus appears as a straight line in FIG. 1. (The same applies for ensuing figures.)
As shown in FIG. 1, the center of the disk is above Track N, and radial and tangential directions are designated with respect to the track segments shown. For the media shown in FIG. 1, the media would spin in a clockwise direction about the center; thus, the segment shown in FIG. 1 is envisioned moving from right to left on the page. The optical spot used to read the tracks would project onto the segment from a fixed tangential position, and would be movable in the radial direction.
As seen, the tracking marks are located at tangential positions A and B for each track. (The tracking marks will also be referred to as "wobble" marks.) The tracking marks are radially located between tracks and on alternate sides of a particular track. Tracking marks A are above each track as shown in FIG. 1, while tracking mark B is below it.
Also shown on the right hand side of FIG. 1 is a "Tracking Error Signal." This graph represents the difference of signal strengths of the tracking marks (A-B) versus radial position of the laser spot. Thus, for a radial position of the laser spot, the Tracking Error Signal strength represents the signal strength read at track position A minus the signal strength read at track position B.
Thus, the Tracking Error Signal is zero when the laser spot is located exactly over a track and increases in magnitude (in a positive or negative direction) as it moves radially away from a track position. Thus, the Tracking Error Signal may be used to adjust the radial position of the spot so that it lies directly above the track. The Tracking Error Signal indicates how much radial adjustment is needed.
The Clock Marks shown in FIG. 1 are used as synchronization marks to indicate (via timing provided by a Phase Lock Loop) where other marks are on the track, thus providing "capture". Each Clock Mark lies directly on a track; thus, when the laser spot is located radially above a track, the signal from the Clock Mark is a maximum.
The Clock Marks and the Tracking Marks are used to generate a second "Cosine Signal," as also shown graphically in FIG. 1. The Cosine Signal graph shown in FIG. 1 is the difference of twice the Clock Mark signal strength minus the sum of the signals of the tracking marks (2C-(A+B)) as a function of radial position of the laser spot. The Cosine Signal is a maximum when the spot is over a track and a minimum exactly between tracks. Thus, the Cosine Signal is 90.degree. out of phase with the Tracking Signal Error.
The Cosine Signal is typically used for deriving the direction of motion of the laser spot during seeks at low radial velocities. A "seek" is where the laser spot is deliberately moved in the radial direction from one track to another. A seek occurs at low velocity when the movement is only over one or a few tracks; thus the velocity of the spot in the radial direction is low. Alternatively, a low velocity seek occurs at the end of a longer multi-track seek. In the middle of such a longer seek, the radial velocity is greater and is then slowed in order to capture the desired track at the end of the seek.
In both cases, determining the direction of motion of the laser spot (referred to alternatively as the "head", which projects the spot) is made difficult by the eccentricity of the tracks relative to the center of the disc. This eccentricity can indicate movement of the head moving in a different direction or at a different speed than it actually is.
Thus, the direction of motion of the optical head (relative to the tracks) is unpredictable at low velocities. The Cosine Signal is used in conjunction with the Tracking Error Signal to provide an accurate determination of the direction of motion. For example, referring to FIG. 1, when the head is moving in the positive radial direction (away from the center of the disc), the leading edge of the digitized Cosine Signal (shown in dashed lines) always occurs when the Tracking Error Signal is positive. Conversely, when the head is moving in the negative radial direction (toward from the center of the disc), the leading edge of the digitized Cosine Signal occurs when the Tracking Error Signal is zero.
Accordingly, if the digitized Tracking Error Signal is used as the input to a flip-flop, with the digitized Cosine Signal used as the gate signal, then movement of the head in the positive radial direction would be indicated by a positive (+1) output of the flip-flop. Movement of the head in the negative radial direction will be indicated by a zero output of the flip-flop.
The tracking format of FIG. 1 presents a number of disadvantages. First, if the pitch of the track (i.e., the space between tracks) is large, or the laser spot is relatively small, the Clock Marks can be missed during seeks. This can cause loss of synchronization, especially when older media having larger pitch is used in future drives having smaller laser spots.
Second, future media will use ever decreasing track pitches. As the track pitch for the media of FIG. 1 is reduced, each track mark moves closer to the neighboring adjacent track. For example, the "B" mark from Track N+2 will approach Track N+3, resulting in larger noise when reading the B mark while tracking at Track N+3. (Of course, an increase in noise will also occur at the A mark on Track N+3 from Tracking Mark A on Track N+4.) Thus, the use of the format of FIG. 1 in future media will suffer as resolution between adjacent A marks and B marks is reduced, resulting in a poor signal to noise ratio in both the Tracking Error Signal and Cosine Signal. In effect, the format limits the track density of the media.
Third, during long seeks, when moving across a large number of tracks, it is desirable to have the head move at a high velocity for much of the seek, in order to minimize the time delay. In some systems, the Tracking Error Signal is sampled in order to count tracks as the spot moves in the radial direction. However, it is seen that the Tracking Error Signal of FIG. 1 completes one complete cycle for each track. This relatively high frequency signal limits the radial seek velocity of such systems; if the spot moves too fast, the signal from an adjacent cycle can give rise to detection of an "alias" signal from an adjacent cycle of the signal, rendering the count of track crossings inaccurate.
Finally, the Tracking Marks in FIG. 1 are not located on the track. Manufacture of such "off-track" marks is more difficult and costly than on-track marks (such as the Clock Marks in FIG. 1).
A number of the difficulties presented by the format of the media of FIG. 1 are improved with the format of the media represented in FIG. 2, which is a segment of the media analogous to that shown in FIG. 1. The format is representative of the one used in the LM6000 media of Philips Laser and Magnetic Storage. Again for the four tracks N to N+3 shown in FIG. 2, the center of the disk is above Track N. As shown in FIG. 2, the media would spin in a clockwise direction about the center; thus, the segment shown in FIG. 2 is envisioned moving from right to left on the page.
The radial position of tracking marks A and B alternate from one side of the track to the other for neighboring tracks. Thus, the B mark for Tracks N and N+1 are both located between Tracks N and N+1. The A marks for Tracks N+1 and N+2 are both located between Tracks N+1 and N+2. Thus, the mark of an adjacent track cannot encroach closer than the tracking mark for the track itself, thus limiting the noise.
The Tracking Error Signal graph again represents the difference of signal strengths of the tracking marks (A-B) versus radial position of the laser spot. Again, the Tracking Error Signal is zero when the laser spot is located exactly over a track and increases in magnitude (in a positive or negative direction) as it moves radially away from a track position. Thus, the Tracking Error Signal may be used to adjust the radial position of the spot so that it lies directly above the track. The Tracking Error Signal indicates how much radial adjustment is needed.
Because the A and B tracking marks are "grouped" on the same side between neighboring tracks, it is seen that the Tracking Error Signal reaches a single maxima or a minima at the half-way point between tracks. The frequency of the Tracking Error Signal is one-half that of the format of FIG. 1. Thus, for systems that use the tracking signal to count tracks during a long seek, the format of FIG. 2 allows the head to move at a higher velocity without an aliasing.
Also shown in the format of FIG. 2 are separate Cosine Marks on each track, located at two tangential positions C and D. The position of the Cosine Marks for adjacent tracks alternate between positions C and D and thus give a Cosine Signal as shown in FIG. 2 that is likewise 90.degree. out of phase with the Tracking Error Signal. In the same manner as explained above with respect to the format of FIG. 1, the Cosine Signal is used in conjunction with the Tracking Error Signal to determine the direction of motion of the head.
Finally, the format of FIG. 2 shows Clock Marks located both on track and between tracks. Having a tracking mark between tracks serves to prevent loss of synchronization for smaller spots and/or large track pitches.
The format of FIG. 2 also has a number of disadvantages. Although the position of the tracking marks serves to reduce noise for neighboring tracks, the pitch of the media still cannot be reduced indefinitely. Although the marks no longer encroach directly on neighboring tracks (as in FIG. 1), a reduction in pitch for the grouping of track marks in FIG. 2 leads to encroachment on adjacent marks. This again gives rise to noise. More fundamentally, it limits the amount the pitch may be reduced, since adjacent track marks will begin to merge.
Further, both the tracking marks and half of the clock marks are off-axis for the format. As already noted, this type of off-axis formatting is both difficult and expensive.